The invention relates generally to medical devices and their use. More specifically, the invention relates to stents for holding vessels such as arteries open to flow, particularly in the regions of bifurcations.
Stents are radially expandable endoprostheses which are typically intravascular implants capable of being implanted transluminally and enlarged radially after being introduced percutaneously. They have also been implanted in urinary tracts and bile ducts. They are used to reinforce body vessels and to prevent restenosis following angioplasty in the vascular system. They may be self-expanding or expanded by an internal radial force, such as when mounted on a balloon.
Stents are generally tubular devices for insertion into tubular vessel regions. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent is biased so as to expand upon release from the delivery catheter.
A vessel having a stenosis may be viewed as an inwardly protruding arcuate addition of hardened material to a cylindrical vessel wall, where the stenosed region presents a somewhat rigid body attached along, and to, the elastic wall. The stenosis presents resistance to any expansion of the vessel in the region bridged by the stenosis. Stenoses vary in composition, for example, in the degree of calcification, and therefore vary in properties as well.
The arcuate geometry of any stenoses present a variation in resistance along the vessel axis to stent outward radial force. Specifically, stenosed vessel resistance is often greatest toward the middle, lessening toward the ends, with a rapid decrease at the start of healthy vessel tissue.
In some instances, as in regions of bifurcation, stenoses are believed to be flow related phenomena, see Chapter 21 of the xe2x80x9cHandbook of Bioengineeringxe2x80x9d (Richard Shaloh and Shu Chin, McGraw-Hill Book Company, 1987) which discusses atherosclerosis at vascular bifurcations.
The left and right common carotid arteries are typical of such vascular bifurcations. These arteries are the principal arteries of the head and neck. Both of the common carotid arteries are quite similar and divide at a carotid bifurcation or bulb into an external carotid artery and an internal carotid artery. In the region of the carotid bulb and the ostium of the internal carotid artery, stenoses present a particular problem for carotid stenting due to the large tapering of the vessel interior from the common carotid artery (both the left and the right) to the internal carotid artery. The region of the carotid bifurcation or bulb happens to be where stenoses most often occur, particularly in the region of the ostium to the internal carotid artery in both of the carotid arteries. Self-expanding stents are generally preferred for carotid stenting due to the anatomical location being subject to external compression.
A conventional self-expanding stent optimally has a length greater than the length of the stenosed region to be kept open. Current stents present a substantially uniform outward radial force and a uniform resistance to compression along their length. Currently, stents do not vary these forces to match vessel geometries or resistances. A constant force stent, i.e., prior art stents, with sufficient force to maintain an open channel within a stenosed vessel and to resist compression, has greater force than necessary in the healthy vessel portion distal to the stenosis. The stent end may thus flare outward, protruding into, and possibly irritating non-stenosed tissue.
Stenoses can occur in vessel regions having asymmetric geometry lying on either side of the stenosis. One example of this is the ostium of an internal carotid artery, having a wide opening converging into a narrower artery. A conventional stent placed in the region of the ostium would provide substantially uniform outward radial force over a non-uniform vessel diameter, that is, the force provided would be greater in a small diameter than in a larger diameter. If this force is properly matched for the smaller vessel region, it is likely less than optimal for the larger region. Conversely, if this force is properly matched for the larger vessel region, it is likely more than optimal for the smaller vessel region.
What would be desirable, and has not heretofore been provided, is a tapered stent capable of providing sufficient force to keep a vessel open within a rebounding stenosis, while providing only necessary force against healthy, non-stenosed vessel regions. What else has not been provided is a tapered stent providing necessary, but only necessary force (outward force and compression resistance) along a stenosis in a vessel region having non-uniform vessel diameter on either side of the stenosis. This is provided by the tapered stents of this invention which exhibit differing radial force, cell size, geometry, flexibility and which provide substantially more constant metal to artery ratio (M/A) over their length. M/A is the ratio of the metal surface area of a stent to the surface area of the vessel or the like that the stent is covering.
The present invention, in a preferred embodiment, includes a self-expanding stent of shape-memory metal having a tubular or cylindrical shaped structure in the unexpanded condition and a tapered tubular or cylindrical structure in the expanded or memorized condition, and in which the radial force varies longitudinally along the length of the stent. Also, its resistance to compression varies with length. Additionally, the cell design making up the stent is closed where force and good plaque coverage and support is required and open where flexibility is required. Additionally, the metal to artery ratio is substantially more constant over the length of the stent when it is expanded. One such stent is constructed of Nickel-Titanium alloy (nitinol). Other shape memory metals may be used. In one embodiment, the stent is constructed and arranged so that the outward radial force is greater in the center and lesser at both ends. In another embodiment, the stent is constructed and arranged so that the outward radial force is greater at one end and less at the opposite end. Such stents are suitable for placement in stenosed and narrowing vessel regions such as the carotid bifurcation and the ostial area associated therewith.
The stents of the invention may achieve a variation in radial force along their length by including in the stent structural elements which intersect at connections having more metal in regions requiring more radial force and less metal in regions requiring less radial force. The amount of intersection metal or strut member metal can be varied by varying the size of the intersection area or the size of the struts. Greater or fewer connectors actually are used to vary the flexibility along the length of the stent more than increasing radial force. In a preferred embodiment, the stent structure is formed by laser cutting a Nitinol tube, leaving a greater strut width and shorter length in regions requiring greater outward radial force and compression resistance.
The struts of the invention are also characterized by the fact that they are constructed and arranged to present a substantially more constant metal to artery ratio over their length in the expanded condition, i.e., expanded to a tapered shape.
The stent structure in a preferred embodiment includes a series of serpentine annular segments which are aligned to provide a tubular structure. The segments are interconnected longitudinally. A desired radial force can be varied by varying the stent strut dimensions in this and other embodiments. In one embodiment, stent regions requiring greater radial force have wider and shorter struts than regions requiring less force. The number of connectors between segments can also be varied for this purpose. It is also obtained by varying strut length and spacing and overall size. Another control is cell design per se. Closed cells provide greater plaque coverage and support than open cells. Closed cells are generally connected to cells in adjoining segments of the stent whereas open cells are not so connected. These factors also provide control over properties such as flexibility and conformability. Cell geometry, i.e., closed and open, is used to provide good plaque support in the region of the stenoses (closed) and less support (open) and more flexibility to either side of the stenoses. Also, closed cell structure may be used to bridge the origin of the external carotid artery or any other vessel side branch opening.
Generally speaking it is desirable to provide a stent of this invention with the aforementioned radial force which is variable over stent length in a predetermined arrangement; cell design which is closed in the area where the stent contacts plaque of a stenoses and more open where the stent contacts healthy vessel tissue; flexibility and conformability which is arranged to vary in a predetermined arrangement over the length of the stent, in both unexpanded and expanded condition.
Stents made in accordance with the present invention can provide an outward radial force more closely matching the local force requirements in a tapered vessel. In particular, the stents provide greater force only where required at a stenosis, without providing too much force in the region of healthy tissue. The stents provide an expanded geometry more closely tailored to the requirements of a tapering vessel region. They are preferably stiff and strong at the proximal large diameter end or middle and weak and more flexible at the distal smaller diameter end to provide strain relief and prevent kinking of the vessel distal to the stent. The proximal end may also be flexible.
A stent of the invention with variable properties along its length also applies to balloon expandable stents that can be used across bifurcations with large diameter change by dilating with a smaller balloon distally and a larger balloon proximally.
This invention is also concerned with a method for treating stenoses in vessel bifurcation regions involving the use of a stent of the type described above.